This generates a docbook phrase with a role attribute, which can be used to classify the phrase. This can be used to mark text for a use that isn't covered elsewhere. The docbook role will generate a html class, which can be used to style text. And the xsl stylesheets can be customized to treat certain roles specially when generating pdfs.

The boostbook css stylesheets, and xsl stylesheets contain support for a limited number of colours that can be used with role. For example if you write:

[role red Text content]

You'll get red text if you're using the boostbook css (for html) or the boostbook xsl for generating pdfs.

The full list of colours that will be available is: